Wide on the rear - better for traction
That all depends on the tire, though. Keep in mind that a wider tire is spreading the same minimal rear weight (600#) over a wider contact patch, therefore lessening the downward force per square inch.
Now, if the wider tire is a better tire than what you had in the narrower size, that may mitigate the loss of downward force.
Also, wider tires may have a better chance of hydroplaning, depending on the tread.